RBSC components are used in many different applications, including kiln furniture, mechanical seals, burner nozzles, radiant tubes, and heating elements. RBSC components are increasingly replacing polysilicon or quartz components in many high temperature applications. Silicon carbide (SiC) is superior for use in such applications because SiC is chemically and mechanically more stable than polysilicon or quartz at high temperatures.
SiC components are used in standard applications—which do not require high purity SiC—and in applications which require high purity and even ultrahigh purity SiC. For example, ultrahigh purity SiC is required to make kiln furniture for use during the manufacture of semiconductor devices. Such components are typically SiC composites composed of a mixture of SiC and silicon (Si) metal rather than 100% SiC, and are termed reaction formed or reaction bonded SiC. Reaction forming of ceramics as an alternative to conventional processing has been of general interest for a long time. Fabrication of these bodies generally involves forming a green body using various standard routes, such as extrusion, slip casting, and dry pressing. The green body consists mostly of SiC, a significantly lesser amount of carbon or graphite, and a polymeric binder. The green body is heat treated to carbonize the binder, and then infiltrated with liquid Si. The Si reacts with the carbon or graphite to form new SiC, which bonds the SiC starting material together (hence the name “reaction bonding”).
The processing of the present invention is similar to RBSC except that the green body, or preform body, contains no SiC and only carbonaceous components—except for impurities that are introduced either in the raw materials or through the fabrication process. The SiC in the final product is formed through the Si+C→SiC reaction. This reaction is highly exothermic, which causes substantial increases in temperature. For typically processed RBSC with only a small amount of carbon as a binding phase, the change in temperature is not extreme. For carbonaceous preforms with no SiC, the mass of the carbon reactant is much greater per unit volume, with even small samples increasing in temperature by more than 500-1000 degrees in a matter of seconds. Temperatures can be so extreme as to vaporize the non-carbon components of the preform resulting in porosity and flaws in the resulting RBSC composite. The porosity may subsequently be filled with molten Si metal (leaving Si “spots”) or, if the sample gets hot enough, the Si will also be vaporized leaving big pores behind as in 2 of FIG. 1. The top sample 2 is much larger and demonstrates significant porosity resulting from extreme temperatures incurred during infiltration. The bottom sample 1 is much thinner and smaller and shows no such flaws, presumably because much lower temperatures were realized during infiltration. There are not many materials that can withstand temperatures in excess of e.g., 2500° C., and thus the greater the amount of impurities in the carbonaceous preforms, the more material that is vaporized. In addition to resulting in inhomogeneous RBSC products, significant thermal stress in the component is incurred, and for bulk pieces above a certain size where the temperature increase is more significant, this can cause cracking or extensive fracture of the reacted materials. Thus, to date there are no commercial bulk RBSC products produced using all-carbon (no SiC) preforms, even though there are significant advantages to be gained by being fabricated in this manner.
The numerous advantages of RBSC even by standard techniques (i.e., silicon melt infiltration in SiC preforms) are well documented, and include low processing temperatures, low raw material costs, near-net-shape tailorability, and low-to-zero shrinkage capability. By using carbonaceous preforms with no SiC, even lower cost fabrication is possible due to the ease of machining (i.e., graphite is much easier to machine than SiC) and from potentially lower temperature processing. The ability to purify the carbonaceous preforms to >99.995% by high temperature heat treatment contributes to higher purity products. SiC preforms cannot withstand the same purification conditions, and thus cannot compete with the purity achievable using all-carbon preforms. Also, higher strength values can be achieved due to the ability to use finer sized powders for preform fabrication. Finer grain size in ceramics directly translates to higher strength.